Biologically active agents for the control of ectoparasites and the like are a necessity of life and are particularly essential to the viability of commercial agriculture and to the welfare of the companion or pet animal community. In practice, these agents persist in their beneficial effects only so long as their active chemical structures can be maintained during exposure to hostile chemical environments (e.g., oxygen and ultraviolet sunlight). Chemical degradation caused by environmental elements has long been a problem with respect to natural insecticides, and in particular pyrethrum, a long-known natural insecticide which is an extract of the chrysanthemum flower.
Pyrethrum is detoxified rapidly by mammalian enzyme systems and is known to be quite safe for application to domestic animals and to plants as well (LD.sub.50 in rats in the range of 500-1,000 mg/kilogram--see Casida, J. E., "Pyrethrum, The Natural Insecticide," Academic Press, 1973). In addition, pyrethrum is not teratogenic, carcinogenic or mutagenic. These characteristics make pyrethrum a highly desirable agent for pest control in numerous applications.
A major disadvantage of pyrethrum is its instability in the environment. Degradation commences immediately upon extraction from its natural source, the chrysanthemum flower, and proceeds at a substantially increased rate after application to a substrate to be treated, such as the coat of an animal, the surface of a plant, household surfaces, etc. The result is a very short effective life (several hours) which precludes pyrethrum as a practical alternative for general long-term pest control. Also, while the synthetic pyrethroids generally exhibit improved stability vis a vis pyrethrum, they too exhibit characteristic degradation--particularly after application in the environment (see Miskus and Andrews, infra, reporting stability on the order of about 4 hours for allethrin.)
In recent years, long-term pest control has generally involved the use of inherently more stable, synthesized insecticides (e.g., chlorinated hydrocarbons, carbamates and organophosphates) which are capable of maintaining sufficient efficacy in pest control for periods of days, or even weeks, after exposure to the environment. Notably, however, these insecticides are characterized by high mammalian toxicities and are frequently so toxic that they cannot be applied more than once in a 7-14 day time period. Indeed, many such insecticides are so toxic that they can never be safely applied for the control of ectoparasites on particularly sensitive companion animals such as cats.
Of particular concern in connection with companion animals is the fact that a major portion of the life cycles of ectoparasites generally does not take place on the host animal. Thus in order effectively to control such pests it becomes necessary to treat the animal's general living environment. Various longacting but toxic synthetic insecticides are frequently used for this purpose, including application indoors to carpeting, bedding and animal sleeping quarters as well as application outdoors to kennels, lawns, patios, etc. It has been observed that pet owners often make indiscriminate use of these synthesized insecticides, and this poses a substantial danger of toxicity to the animals intended to be protected as well as to the owners themselves. Moreover, the foregoing danger is frequently compounded by pet owner efforts to control other forms of pests by applying the same or similar toxic actives to the pet's general environment.
In view of such problems and with an increasing interest in safety and freedom from toxicity, significant efforts are currently being directed at replacing toxic synthesized insecticides with biodegradable pyrethrum and its synthetic counterparts (i.e., synthetic pyrethroids).